tetraquark or dual-meson? —

LHC makes clear identification of a weird particle made of four quarks

What kind of particle is it? We don't know yet.

Quarks are gregarious particles, but only within limits. Protons, neutrons, and other baryons are made up of three quarks, while unstable particles called mesons are composed of a quark-antiquark pair. (An antiquark is the antimatter partner to a quark.) Nothing with more than three quarks in a single particle has been found in nature, at least under ordinary circumstances.

Since 2008, however, researchers at CERN in Europe and at Belle in Japan found hints of a four-quark particle. Those hints were confirmed today by physicists at the LHCb experiment at CERN. To discover and characterize the particle, researchers sifted through 25,000 decays of mesons resulting from more than 180 trillion collisions at the Large Hadron Collider. This object they studied, known affectionately as Z(4430)-, has provided the first unambiguous measurement of a four-quark particle; earlier experiments also provided hints about another candidate, the Zc(3900)+.

With that much data, physicists were able to determine the composition of the Z(4430)-: it consists of a charm quark, a charm anti-quark, a down quark, and an up antiquark. The "4430" part of the name indicates its mass: 4,430 million electron-volts, which a little more than four times the mass of a proton (938 million electron volts). The combination of quarks gives the Z(4430)- a negative electric charge, hence the "-" in the label. The particle is highly unstable, so none of them are expected to be seen in nature.

The remaining major question is what kind of particle this is. Could Z(4430)- be a tetraquark—a new class of hadron made of four quarks—or is it a kind of fusion between two mesons? Either way, it's not easily explained by the usual quark behavior, and it's an exciting new type of object.

Perhaps that's why it's so unstable. Perhaps this is the particle physics equivalent of radicals in combustion - things that are totally unstable and violate all the normal "rules" of composition that we normally associate with molecules.

On another note, I'm glad we've finally found something entirely new in the LHC data. We know our understanding of physics isn't yet complete (quantum gravity anyone?) but it was getting depressing just getting confirmation after confirmation of the Standard Model.

That was my first question as well. A down quark paired with an up anti-quark should have color, while the charmed quark anti-quark pair would be colorless. It follows that the 4 together should have color. What happened to quantum chromodynamics?

From what I understand of Quantum Chromodynamics (and I admit to knowing very little) the end result is always colourless be it baryon or meson. It is possible that the up antiquark and the down quark are of opposite colours so as to be colourless since the charm quark/charm antiquark pair would be colourless by definition.

First the Higgs Boson and now this, the LHC is proving to be a worthy investment and who knows when it may help shape a Theory Of Everything.

ETA: The colour property is as inviolable as Pauli's exclusion principle so the end result "must" be colourless.

That was my first question as well. A down quark paired with an up anti-quark should have color, while the charmed quark anti-quark pair would be colorless. It follows that the 4 together should have color. What happened to quantum chromodynamics?

I think everyone here is conflating color with flavor. Each quark can have any of the three colors and each antiquark can have any of the three anticolors. The down quark could be blue and the anti-up quark anti-blue, making the pair colorless.

That was my first question as well. A down quark paired with an up anti-quark should have color, while the charmed quark anti-quark pair would be colorless. It follows that the 4 together should have color. What happened to quantum chromodynamics?

I think everyone here is conflating color with flavor. Each quark can have any of the three colors and each antiquark can have any of the three anticolors. The down quark could be blue and the anti-up quark anti-blue, making the pair colorless.

That was my first question as well. A down quark paired with an up anti-quark should have color, while the charmed quark anti-quark pair would be colorless. It follows that the 4 together should have color. What happened to quantum chromodynamics?

So I have not one, but two blind-idiot questions, because I am not a particle physicist:

1) Why would a down-quark paired with an up-anti-quark have color?2) Why would a charmed quark/anti-quark pair have no color?

What does it mean for a subatomic particle to have "color" or "flavor"? Clearly it is not the same as the properties of light (as detected by our eyes) and molecular compounds (as detected by our tongues).

I think everyone here is conflating color with flavor. Each quark can have any of the three colors and each antiquark can have any of the three anticolors. The down quark could be blue and the anti-up quark anti-blue, making the pair colorless.

Yeah, but an anti-up isn't the same flavour as a down, from what I understand. This particle appears to both fruity flavoured and technicolor! I propose we call it the Fruit Loop (add "of doom" if you wish).

A Baryon has three quarks, one of each colour and so ends up neutral / whiteAn antibaryon has three quarks, one of each anti-colour and so agains ends up neutral / whiteA Meson has two quarks, one colour, one the equivalent anti colour and so again ends up neutral / white

so this new particle can be a form of dual-meson with two quarks of colour and two of anti colour - resulting in neutral.

What does it mean for a subatomic particle to have "color" or "flavor"? Clearly it is not the same as the properties of light (as detected by our eyes) and molecular compounds (as detected by our tongues).

They're intrinsic properties that can be different among different particles, and they affect how the particles behave and interact. They're like "spin" or "charge," but not really good analogs with real-world stuff like those properties' names are.

On another note, I'm glad we've finally found something entirely new in the LHC data. We know our understanding of physics isn't yet complete (quantum gravity anyone?) but it was getting depressing just getting confirmation after confirmation of the Standard Model.

Agreed. Confirming things that you expect is certainly nice, but on the other hand...

"The most exciting phrase to hear in science, the one that heralds new discoveries, is not Eureka! (I found it!) but rather, 'hmm... that's funny...'"

I think everyone here is conflating color with flavor. Each quark can have any of the three colors and each antiquark can have any of the three anticolors. The down quark could be blue and the anti-up quark anti-blue, making the pair colorless.

Yeah, but an anti-up isn't the same flavour as a down, from what I understand. This particle appears to both fruity flavoured and technicolor! I propose we call it the Fruit Loop (add "of doom" if you wish).

I prefer Fruity Particles, with the box featuring the Jetsons.

Edit: Oh come on guys, apparently there aren't any Fruity Pebbles fans here? Personally I'd get a kick out of seeing a rebrand of Fruity Pebbles into Fruity Particles....

What does it mean for a subatomic particle to have "color" or "flavor"? Clearly it is not the same as the properties of light (as detected by our eyes) and molecular compounds (as detected by our tongues).

Flavor = type of quark - up, down, strange, charm, top, bottom. In general it means "type of [insert family of particles]"

Color = analogous to electric charge, but for the Strong Nuclear Force, and more complicated because the strong force is more complicated. Unlike electric charge which is simply positive or negative, color charge can be one of three types we call "red", "green", or "blue" (or anti- those colors for anti-quarks, which can be thought of as "cyan", "magenta", and "yellow"). When you combine red, green, and blue you get "white" which is neutral. Or any other combination that results in white.

So the term "color" actually works pretty well as an analogy.

Unlike the electric charge, the color charge of a quark can change. The gluon, which is the photon-like particle that mediates the strong force, carries color charge itself and can instigate these color changes. This is part of why the strong force is so strong, and complicated.

The reason people are asking if this would result in a color-neutral particle is because we've never observed a net color charge before, and because of how the strong force acts it is considered impossible. Any bare quark would have such a strong potential (strong force, yah?) that the energy would cause other quarks and such to spontaneously pop into existence and form a new, neutral hadron ( = thingie what is made of quarks and gluons). If you ever hear someone talking about "jets" in the LHC, this is what they mean -- when they smash a proton apart, any quarks that might have gone flying off on their own instead form new hadrons (which may be protons, neutrons, or exotic things like this article talks about).

On another note, I'm glad we've finally found something entirely new in the LHC data. We know our understanding of physics isn't yet complete (quantum gravity anyone?) but it was getting depressing just getting confirmation after confirmation of the Standard Model.

Agreed. Confirming things that you expect is certainly nice, but on the other hand...

"The most exciting phrase to hear in science, the one that heralds new discoveries, is not Eureka! (I found it!) but rather, 'hmm... that's funny...'"

Even more exciting is "No, no, that's not possible- Everyone, run for your lives!"

On another note, I'm glad we've finally found something entirely new in the LHC data. We know our understanding of physics isn't yet complete (quantum gravity anyone?) but it was getting depressing just getting confirmation after confirmation of the Standard Model.

Agreed. Confirming things that you expect is certainly nice, but on the other hand...

"The most exciting phrase to hear in science, the one that heralds new discoveries, is not Eureka! (I found it!) but rather, 'hmm... that's funny...'"

Even more exciting is "No, no, that's not possible- Everyone, run for your lives!"

---- as the scientists run away from an experiment run amok with the lab /literally crumbling away at their heels. They may even be required to outrun an explosion or two.

A Baryon has three quarks, one of each colour and so ends up neutral / whiteAn antibaryon has three quarks, one of each anti-colour and so agains ends up neutral / whiteA Meson has two quarks, one colour, one the equivalent anti colour and so again ends up neutral / white

so this new particle can be a form of dual-meson with two quarks of colour and two of anti colour - resulting in neutral.

Hadrons are particles made of quarks.

It is pretty heavy for a dual meson. Then again, I don't have much experience with dual mesons.

What does it mean for a subatomic particle to have "color" or "flavor"? Clearly it is not the same as the properties of light (as detected by our eyes) and molecular compounds (as detected by our tongues).

Flavor = type of quark - up, down, strange, charm, top, bottom. In general it means "type of [insert family of particles]"

Color = analogous to electric charge, but for the Strong Nuclear Force, and more complicated because the strong force is more complicated. Unlike electric charge which is simply positive or negative, color charge can be one of three types we call "red", "green", or "blue" (or anti- those colors for anti-quarks, which can be thought of as "cyan", "magenta", and "yellow"). When you combine red, green, and blue you get "white" which is neutral. Or any other combination that results in white.

Well, the electromagnetic force is in high energy physics combined with the weak nuclear force that has two types of charges. Giving the electro-weak force 3 different charges just like the chromatic (strong nuclear) force.

On another note, I'm glad we've finally found something entirely new in the LHC data. We know our understanding of physics isn't yet complete (quantum gravity anyone?) but it was getting depressing just getting confirmation after confirmation of the Standard Model.

Agreed. Confirming things that you expect is certainly nice, but on the other hand...

"The most exciting phrase to hear in science, the one that heralds new discoveries, is not Eureka! (I found it!) but rather, 'hmm... that's funny...'"

Even more exciting is "No, no, that's not possible- Everyone, run for your lives!"

Perhaps that's why it's so unstable. Perhaps this is the particle physics equivalent of radicals in combustion - things that are totally unstable and violate all the normal "rules" of composition that we normally associate with molecules.

On another note, I'm glad we've finally found something entirely new in the LHC data. We know our understanding of physics isn't yet complete (quantum gravity anyone?) but it was getting depressing just getting confirmation after confirmation of the Standard Model.

Unfortunately, it looks as though it doesn't really bring anything new to the table. There doesn't look to any new physics here, which is what physicists want. But it's interesting on its own.

Electrons, protons, neutrons I get. But the other stuff appears to only exist if you break the others. Doesn't that imply they don't actually exist in nature? Kinda like if I break a glass and start naming the broken pieces. The broken pieces didn't exist until I broke the glass.

Electrons, protons, neutrons I get. But the other stuff appears to only exist if you break the others. Doesn't that imply they don't actually exist in nature? Kinda like if I break a glass and start naming the broken pieces. The broken pieces didn't exist until I broke the glass.

The fundamental particles that make up hadrons (not electrons; those are fundamental particles too) are all of a different character than the hadrons they make up. If you break a sheet of glass, you still have glass. It's the same stuff, just smaller. That's not the case with quarks and gluons vs. protons and neutrons.